Method of reducing solid arcing product build up between electrical contacts in pressurized fluid ambients

ABSTRACT

A method of substantially reducing the build-up of solid arcing products between contacts for making and breaking electrical circuits including the steps of adjusting the contacts to an arbitrary opening distance at which arcing will occur, placing the contacts in the dielectric fluid under greater than atmospheric pressure, opening and closing the contacts at a fixed operating circuit voltage to determine if the solid arcing product build-up forms, and readjusting the contact opening distance to minimize the value of the parameter (V/N)2/d where V is the operating circuit voltage, N is the number of sets of serially connected contacts and d is the distance between the contacts when in the fully open position. The value of the parameter may be further reduced by distributing V across a plurality of sets of such contacts connected in series and provided to be simultaneously actuated.

United States Patent [191 Pocock June 28, 1974 METHOD OF REDUCING SOLID ARCING PRODUCT BUILD-UP BETWEEN ELECTRICAL CONTACTS lN PRESSURIZED FLUID AMBIENTS [75] Inventor: Walter E. Pocock, Baltimore, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

22 Filed: Dec. 20, 1972 211 Appl. No.: 316,778

[52 U.S.Cl. ..I200/24'9',29/622 511 Int. Cl. HOlh 1/34 58 Field of Search 29/630 R, 622; 200/166 R, ZOO/166B [56] References Cited OTHER PUBLICATIONS Peek et al., Switching Relay Design, 1955, p. 26.

ADJUST CONTACTS TO ARBITRARY OPENING DISTANCE PLACE CONTACTS IN DIELECTRIC FLUID UNDER GREATER THAN ATMOSPHERIC PRESSURE OPEN AND CLOSE CONTACTS AT OPERATING VOLTAGE READJUST CONTACTS TO MINIMIZE Primary ExaminerRobert K. Schaefer Assistant Examiner-William]. Smith [57] ABSTRACT A method of substantially reducing the build-up of solid arcing products between contacts for making and breaking electrical circuits including the steps of adjusting the contacts to an arbitrary opening distance at which arcing will occur, placing the contacts in the dielectric fluid under greater than atmospheric pressure, opening and closing the contacts at a fixed operating circuit voltage to determine if the solid arcing product build-up forms, and readjusting the contact opening distance to minimize the value of the parameter (V/N) /d where V is the operating circuit voltage, N is the number of sets of serially connected contacts and d is the distance between the contacts when in the fully open position. The value of the parameter may be further reduced by distributing V across a plurality of sets of such contacts connected in series and provided to be simultaneously actuated.

3 Claims, 1 Drawing Figure DISTRIBUTE OPERATING VOLTAGE ACROSS A PLURALITY OF SETS OF CONTACTS PAIENIEflJuuza I974 ADJUST CONTACTS TO ARBITRARY OPENING DISTANCE PLACE CONTACTS IN DIELECTRIC FLUID UNDER GREATER THAN ATMOSPHERIC PRESSURE OPEN AND CLOSE CONTACTS AT OPERATING VOLTAGE READJUST CONTACTS TO MINIMIZE DISTRIBUTE OPERATING VOLTAGE ACROSS A PLURALITY OF SETS OF CONTACTS 1 METHOD OF REDUCING SOLID ARCING PRODUCT BUILD-UP BETWEEN ELECTRICAL CONTACTS IN PRESSURIZED FLUID AMBIENTS The invention described herein may be used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates generally to circuit makers and breakers and more particularly to those of the mechanical type.

It was common, though not universal, practice on early deep submergence vehicles to put the bulk of the electrical switching gear inboard. Due to increased power loads and the desirability of placing circuit interrupting devices outside the pressure hulls of deep submersibles, there were two choices for protective housings for such devices. First, a hard shell enclosure filled with air or an inert gas under normal atmospheric pressure and built to withstand the external deep ocean pressure could be used. Second, a thin wall pressure compensated chamber filled with a dielectric fluid could be used. In the thin wall chambers, the switching or circuit-interrupting device is immersed in the dielectric fluid and is subject to the pressure from the external ocean environment.

In the first case, the hard shell enclosures are disadvantageous in that they add bulk and weight which are critical factors in deep submergence vehicles. Also, the removal of heat from inside the hard shell and the trapping of gaseous degradation products from the electrical insulation and other materials cause additional problems.

In the second case, the method is to immerse the electrical devices in a fluid having good dielectric properties, in the pressure-compensating chamber. The latter is a thin-walled enclosure of sufficient flexibility, or with a bellows attachment, to allow for compression of the fluid at the deep-ocean pressures. The fluids used or proposed for use are either petroleum-derived or silicone liquids having a specific gravity less than 1.0, thus providing a meaure of buoyancy.

A number of unexpected electrical contact failures on circuit-interrupting devices occurred in these early applications. Solid products, later identified as carbon in the case of petroleum-derived fluids and carbon plus silica (silicon dioxide, SiO in the case of silicones, which had resulted from arcing in the fluids during circuit interruption, had bridged the contacts, causing a short circuit in the open position. The objectionable deposits were dubbed clinkers. These failures, which were not too clearly understood, and for which there was no prospect of an immediate solution, caused an understandable reaction among those involved in designing and operating deep submersibles. In at least one case, it was decided to use hard shell enclosures for housing circuit-interrupting equipment on a vehicle then in the planning stage. This approach involves other problems. notably a large increase in weight, sealing and making penetrations through the enclosure,

and poor heat dissipation from the equipment inside the hard shell.

To date, various stock circuit interrupting devices have been adapted, sometimes with design modification for operation in fluids on deep submersibles, often with highly unsatisfactory results. At one time, there were considerable doubts about using pressure compensating fluids because of the history of failure. However, the advantages of bulk and weight savings, few or no penetration problems, and good heat transfer make this an attractive approach.

SUMMARY OF THE INVENTION In accordance with this invention, contact failure problems in circuit interrupting devices can be overcome, making the pressure compensating fluid ap proach not only practical but probably the preferred direction to take. It has been found, inaccordance with this invention, that the build-up of solid arcing products between contacts of a circuit interrupter in dielectric fluids under greater than atmospheric pressure can be substantially reduced by a method of minimizing the value of the parameter (V/N) /d where V is the operating circuit voltage, N is the number of sets of serially connected contacts and d is the distance between the contacts when in the fully open position.

STATEMENT 6F THE OBJECT OF THE INVENTION a Accordingly, the primary object of this invention is to provide a method of substantially reducing the buildup of solid arcing products between contacts in dielectric fluid under greater than atmospheric pressure;

It is also an object of this invention to reduce such build-up by minimizing the value of the parameter (V/N) /d where V is the operating circuit voltage, N is the number of sets of contacts and d is the distance between the contacts when in the fully open position.

It is a further object of this invention to reduce such build-up by adjusting the opening distance to determine a suitable opening distance necessary to substantially reduce the solid arcing producing build-up between the contacts; and

It is still a further object of this invention to reduce the value of the parameter by distributing V across plural sets of 'such contacts connected in series and provided to be simultaneously actuated.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing wherein:

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a diagrammatic illustration of the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT observed on deep-diving vehicles. The following additional observations were made:

Contact failure in fluids under pressure by clinker formation, often occurs in fewer than 10 interruptions.

Contact life under pressure (stated as the number of interruptions to failure) decreases with:

increasing open-circuit voltage;

increasing current;

increasing fluid viscosity; and

increasing pressure.

The'failure of contacts under pressure depends not only on the amount of solid product formed but also on how readily it deposits between the contacts in preference to dispersing in the fluid. This is believed to be related to the formation of bubbles of gaseous arc breakdown products. At atmospheric pressure the motion of the bubbles tends to sweep the solid particles from the contact zone and causes them to disperse in the fluid. At higher pressures the bubbles are smaller and denser and therefore slower moving, or they may not form at all because of their solubility in the fluid at high pressure. Thus the solids-dispersing action is lost. It has also been suggested that electrical charges on the particles, under the influence of the electrical field of the opening contacts, may partially account for the bridging of the contacts by the solids. The clinker in one case, formed by interrupting 100 amperes at 120 volts dc, in 1 cs silicon fluid at 13,500 psi, hasbeen tentatively identified as a mixture of silica and carbon. The silica (white in color) completely coats the outside of the deposit, which when broken open, is seen to have a core of carbon. This may very well indicate the migration of charged carbon and silica particles in opposite directions.

Among the various factors that have been investigated in relation to the clinker failure problem, two controllable variables appear to be highly critical in their effect on contact life. The first of these is the voltage across the contacts when completely open. For a particular set of contacts, in a given dc circuit, in a given medium (gaseous or liquid) and at a fixed current, the energy k of the-circuit-interrupting arc falls off sharply 'as open-circuit voltage is decreased. This is When the voltage is halved, the arc energy decreases to one-fourth of its original value or less. Thus, arc energy varies as the square of the open circuit voltage, or

greater. It has already been stated that contact life under pressure decreases with increasing open-circuit voltage. The magnitude of this decrease is also fairly larger for a given increase in voltage. It is not certain, however, that voltage affects contact life only by virtue of its effect on arc energy.

It can be seen that in going from, say a 60-volt to a 120-volt system on a vehicle, the problem of electrical contact failure in pressure-compensating fluids increases sharply. The principle just discussed can be utilized by designing circuit-interrupting devices with multiple sets of contacts in series. Ideally, if all the sets of contacts opened simultaneously, the total voltage would be distributed uniformly over the individual sets during the arcing process. Thus, using a device having four sets in series in a volt circuit, the voltage per set of contacts would be 30 volts. In practice, the contacts do not open exactly simultaneously, and the resistance across the arc itself varies from one set of contacts to another; but it has been shown that a very good approach to the ideal condition can be made without the need of highly critical adjustments. There is,'of course, a practical limit to the number of sets of contacts than can be used in series, beyond which the weight and size of the device increases out of proportion to the benefits obtained. For applications being considered here, with maximum voltages of 120 volts and maximum currents of 500 amperes in normal operation, four or possibly six sets are felt to be a reasonable limit based on experience thus far.

Control of the parameter (V/N) /d has been found to have a significant effect in lessening the tendency of clinker failure. Such control may be accomplished by controlling the operating voltage at the are between each set of contacts.

The second controllable variable that is highly critical in its effect on contact life under pressure is contact spacing, that is, the space between contacts in the fullopen position. The typical effect is seen in the data of Table 2.

TABLE 2 0 Effect of Contact Spacing on Contact Life, in 1 cs Silicone Fluid,

13,500 psi, 120 volts dc, at Two Current Values Contact Contact Life, Number Test Current Spacing, of Interruptions to Run Amperes Inches C1inker Failure 3 0.25 10,000 (no failure) 6 0.25 10,000 (no failure) 3 shows that contact life decreases with an increase in this parameter. (Note, however, that certain runs were arbitrarily stopped after 5000 interruptions even though there was no clinker failure at that point.) The relationship between contact life and voltage at the contacts can be obtained by a more detailed inspection of these data. Comparing test numbers A-4 or A-5 to test number A-6, which were all run at the same contact spacing d, tests A-4 and A-5 did not show failure in 5,000 interruptions; test A-6, with double the voltage of tests A-4 and A-5, showed failure in 1219 interruptions. Thus, contact life decreased sharply when TABLE 3 Results of Tests in Two Liquids Under 13,500 PS1 Pressure, Breaking 100 Amperes DC, at a Total Open-Circuit Voltage of 120 Volts the voltage was doubled, with (1 being held constant.

Since it has already been shown that. contact life tends to increase as the value of d increases, the following comparisons can also be made:

1. Between tests A-3 and B-lb, contact life decreased markedly when the voltage was doubled, even though d was slightly more than doubled.

2. Between tests A-3 and B-2 or B-3, contact life decreased sharply when voltage was doubled, even though d was increased.

3. Between tests A-4 or A-5 and B-4, contact life decreased when voltage was quadrupled even though d was more than quadrupled.

4. Between tests A-6 and B-5, contact life decreased sharply when voltage was doubled, even though d was more than doubled.

Thus, the data of Table 3 show that a relationship exists between the voltage at the contact break, the spacing between the contacts, and the number of interruptions before failure occurs. However, the numerical weight to be given to a in its effect on contact life is not the same as that to be given to V/N in its effect on contact life, as shown most clearly by the previous comparison of test A-6 to test A-5. In this comparison, doubling the voltage caused contact life to decrease even though d (which has an effect counter to that of V/N) was more than doubled. Likewise, comparing test A4 or A-5 to 8-4, quadrupling the voltage caused contact life to decrease, even though d was more than quadrupled.

In analyzing the data of Table 3, it was found empirically that a correct numerical weighting would be to use V/N to the second power and d to the first power, resulting injthe parameter (V/N) /d. If, now, contact life is compared to this parameter, it is seen that contact life in each liquid increases uniformly as (V/N) /d decreases, as is clearly shown by Table 3.

In order to use the paramter, a series of runs is made "at different values of (V/N) /d at constant electrical current, each run continuing until failure occurs. The data are then plotted for each fluid, resulting in a graph of number of interruptions plotted against (V/N) /d for a particular fluid and electrical current.

The graph will indicate a limiting value of the parameter below which no clinker failure is expected to occur; the circuit interrupting device is then designed to have a parameter of this value or less, resulting in a circuit that will meet or exceed the required number of interruptions. Alternatively, if the data will permit, the total number of interruptions required for a particular circuit and application is estimated and the corresponding value of the parameter is obtained; this will allow the design of a circuit-interrupting device that has a minimum operational lifetime that is sufficient for a given application but does not have an unlimited life. Such a design procedure is necessary where size or weight limitations are extremely critical, since an increased lifetime usually requires increased weight and- /or volume.

The present method of substantially reducing the build-up of solid arcing products between electrical contacts at a given operating voltage in dielectric fluids under greater than atmospheric pressure includes adjusting the contacts to an arbitrary opening distance at which arcing will occur, placing the contacts in the dielectric fluid, under greater than atmospheric pressure, opening and closing the contacts at a fixed operating circuit voltage to determine if the solid arcing product build-up forms, and readjusting the contact opening distance to minimize the value ofthe parameter (V/N) /d where V is the operating circuit voltage, N is the number of sets of serially connected contacts and d is the distance between the contacts when in the fully open position. The value of the parameter may be further reduced by distributing the operating circuit voltage V across a plurality of sets of such contacts connected in series and provided to be simultaneously actuated. The use of such multiple contact breaks accomplishes distributing V across plural sets of contacts at the breaking of the circuit as long as the plural sets of contacts are substantially simultaneously actuated.

If a number of sets of contacts, or contact breaks, in series are opened simultaneously, the circuit voltage is theoretically distributed substantially equally over the individual sets of contacts. The ideal condition of all the sets of contacts opening exactly simultaneously is not likely in practice and therefore the reduction in arc energy will not be as great as desired. However, the ideal condition can be approached to a highly useful degree in that a partial distribution of voltage among the multiple breaks and a lowering of arc energy are obtained. A good criterion of satisfactory arc distribution on a multi-break contactor has been found to be the appearance of visible arcing at all breaks during contact opening, even though the intensities of the individual arcs may be unequal.

. In the selection or design of a contactor, the basic features such as: single versus double pole, single versus double throw, latching circuits versus spring return of armatures, and normally open versus normally closed contacts are determined primarily by the requirements of the circuit. Substantially equally distributing the circuit voltage'across the plural sets of contactors necessitates either using a plurality of singlepole contactors, or a single device having a plurality of poles. A plurality of single-pole contactors will not be satisfactory since their contacts may not open simultaneously, or nearly simultaneously, so that the possible benefit of the multiple breaks is not obtained. Furthermore, the critical factors of total bulk and weight of a plurality of single-pole contactors may be considerably more than that of a single multiple pole device. Therefore, a single multiple pole device is preferred.

As has been stated, the use of electrical contacts in a dielectric fluid under greater than atmospheric pressure is known, but, the gist of the present invention lies in the reduction of the build-up of solid arcing products between such contacts due to control of the parameter (V/N) /d such that the value of the parameter may be minimized either by adjusting the opening distance d between the contacts and/or by distributing the circuit connected contacts having a contact gap d, the method of reducing the formation of solid particles between the contacts which comprises: reducing the value of the parameter (V/N) /d, wherein V is the circuit voltage, N is the number of sets of contacts, and d is the contact gap spacing in inches, to less than an experimentally predetermined value based on the operational fluid, fluid pressure, and electrical current.

2. The method of claim 1 wherein said reduction is accomplished by increasing the contact gap d.

3. The method of claim 5 wherein said reduction is accomplished by increasing the plurality N of sets of contacts. 

1. In an electrical circuit immersed in a dielectric fluid capable of producing solid particles as a result of arcing between contacts, said circuit comprising a source of voltage V and a plurality N of sets of serially connected contacts having a contact gap d, the method of reducing the formation of solid particles between the contacts which comprises: reducing the value of the parameter (V/N)2/d, wherein V is the circuit voltage, N is the number of sets of contacts, and d is the contact gap spacing in inches, to less than an experimentally pre-determined value based on the operational fluid, fluid pressure, and electrical current.
 2. The method of claim 1 wherein said reduction is accomplished by increasing the contact gap d.
 3. The method of claim 5 wherein said reduction is accomplished by increasing the plurality N of sets of contacts. 